De acordo com Wikipedia , tudo é criptografado, exceto as conexões HTTP.
The connection between the Google Talk client and the Google Talk server is encrypted, except when using Gmail's chat over HTTP, a federated network that doesn't support encryption, or when using a proxy like IMLogic.[8][9] End-to-end messages are unencrypted.[8] Google plans to add support for chat and call encryption in a future release.[10] Some XMPP clients natively support encryption with Google Talk's servers. It is possible to have end-to-end encryption over the GTalk network using OTR (off-the-record) encryption.
LifeHacker tem um artigo sobre como isso pode ser alcançado usando o SSH.